A semiconductor device such as a LSI (Large Scale Integrated circuit) or a MOS (Metal Oxide Semiconductor) transistor, a liquid crystal display (LCD), an organic EL (Electro Luminescence) device, and the like are manufactured by performing a process such as etching, CVD (Chemical Vapor Deposition) or sputtering on a processing target substrate. For the etching, CVD and sputtering processes, methods using plasma as an energy supply source, i.e., plasma etching, plasma CVD and plasma sputtering are employed.
Here, a technique related to a plasma processing apparatus configured to perform a process by using plasma is described in International Patent Publication No. WO2004/068917 (Patent Document 1). According to Patent Document 1, a magnetron is used as a high frequency oscillating source for generating a microwave. Since the magnetron is of a relatively low price and is capable of outputting a high power, the magnetron is effectively used as the oscillating source for generating a microwave.
In the plasma processing apparatus described in Patent Document 1, an automatic matching device is provided. The automatic matching device includes a load matcher composed of a multiple number of stubs protruded from an inner wall surface of a cylindrical waveguide in a radial direction thereof. Each stub is a cylindrical body made of a metal or a dielectric material. A reactance of the stub may vary depending on its protruding length, i.e., a length protruded from the inner wall surface of the cylindrical waveguide in a radial direction thereof, and a reactance within the cylindrical waveguide may also vary depending on the reactance of the stub. The protruding length of the stub can be easily varied by a driving device of the load matcher.
A technique of generating a microwave using a magnetron is also described in Japanese Patent Publication No. 4115618 (Patent Document 2). Patent Document 2 describes a magnetron output control method. In this method, a standing wave of a microwave is split into a traveling wave that propagates from the magnetron toward a plasma load and a reflected wave reflected from the plasma load. By detecting the traveling wave and the reflected wave, a signal corresponding to a power of the traveling wave and a signal corresponding to a power of the reflected wave are detected. If the signal corresponding to the power of the reflected wave is equal to or higher than a preset reference value, an output control of the magnetron is switched from a control of regulating the power of the traveling wave into a constant value to a control of regulating an anode current to a constant value. Meanwhile, if the signal corresponding to the power of the reflected wave is less than the preset reference value, the output control of the magnetron is switched from the control of regulating the anode current to the constant value into the control of regulating the power of the traveling wave to the constant value.    Patent Document 1: International Patent Publication No. WO2004/068917    Patent Document 2: Japanese Patent Publication No. 4115618
According to Patent Document 1, a controller provided in the plasma processing apparatus calculates a load side impedance from an output signal of a detector and calculates the protruding length of the stubs that allows impedance matching between a power supply side and a load side. Then, the controller adjusts the protruding length of the stubs by operating the driving device of the load matching unit based on the calculation result, so that the impedance matching between the power supply side and the load side can be performed. Further, a 4E tuner as a matching device is provided between the magnetron and the load side, and the impedance matching is performed by adjusting positions of four movable short-circuit plates provided in the 4E tuner. To elaborate, the movable short-circuit plates are arranged at certain positions in the cylindrical waveguide. A position of each movable short-circuit plate, specifically, a position in a radial direction thereof is adjusted by rotating a ball screw by a motor, so that impedance matching is performed. Further, by detecting, e.g., whether an emission intensity of a wavelength of a certain element, which is included in plasma generated in a processing vessel, is stable or not, it can be investigated whether the reactance matching is performed or not, or whether the generated plasma is stable or not.
As for the control of adjusting the protruding length of the stubs or the positions of the movable short-circuit plates, it may take a long time before the stubs or the movable short-circuit plates are actually moved and arranged at the appropriately calculated positions after a control signal is outputted. That is, the control of adjusting the protruding length of the stubs or the positions of the movable short-circuit plate may lack rapid responsiveness. Under this circumstance, the generated plasma may be unstable in the plasma processing apparatus configured to generate a microwave by using the magnetron and to generate plasma by using the microwave.
To elaborate, as the microwave generated by the magnetron propagates in the waveguide, a traveling wave is generated. This traveling wave is monitored by detecting a power of the traveling wave that propagates within the waveguide. Here, a reflected wave reflected from a load side may be generated depending on a kind of a gas supplied into the processing vessel or a variation in a pressure within the processing vessel. This reflected wave is also monitored by detecting a power of the reflected wave, and there may arise occasions when the power of the reflected wave increases rapidly. In such occasions, it may be difficult to appropriately respond to the rapid increase of the power of the reflected wave just by adjusting the protruding length of the stubs or the positions of the movable short-circuit plates. That is, in performing the matching of the power of the reflected wave, it may take a long time to start the alignment of the positions of the movable short-circuit plate. As a result, before the position alignment of the movable short-circuit plates is started, a phenomenon, called spike, that plasma emission is darkened momentarily or a phenomenon, called hunting, that plasma emission is darkened or brightened repeatedly may occur repeatedly. If such phenomenon as hunting occurs, it may take a long time until stable plasma is generated, which is undesirable. Further, in the control of the plasma processing apparatus, if the detected power of the reflected wave exceeds a set threshold value or if an emission intensity at a wavelength of a certain element being measured is below a reference value, it may be determined that the plasma processing apparatus is performing an inappropriate plasma process, and the plasma process may be stopped.
According to Patent Document 2, the output control of the magnetron may be switched between the control of regulating the power of the traveling wave to a constant value and the control of regulating the anode current to a constant value by using the reference value for the power of the reflected wave as a threshold value. According to this control manner, however, a power supplied from the magnetron to the load side may be changed. As a result, when the plasma process is performed by the plasma generated in the plasma processing apparatus, the non-uniform plasma process may be performed.